The present invention generally relates to a non-volatile semiconductor memory device. More particularly, the present invention relates to a non-volatile semiconductor memory device having a booster including a capacitor.
Electrically writable and erasable non-volatile semiconductor memory devices such as flash memory devices and EEPROM (Electrically Erasable and Programmable Read Only Memory) devices use two different voltages, one for read operation and one for write and erase operations. For example, a power supply voltage of 5 V is used for read operation, and a voltage of 12 V, i.e., the power supply voltage boosted by a booster, is used for write and erase operations.
In a conventional non-volatile semiconductor memory device, a booster is formed by booster cells connected in parallel. Each booster cell is formed by a switching element and a capacitor. The capacitor in the booster has a square shape of about 200 μm×about 200 μm and is formed by plate-like upper and lower electrodes facing each other with a capacitance insulating film interposed therebetween. The capacitor having a larger capacitance provides a greater boosting effect.
On the other hand, with recent progress in reduction in size and voltage of a semiconductor device, a reduced power supply voltage is often used especially in a non-volatile semiconductor memory device incorporating a logic circuit. Therefore, the booster must conduct the boosting operation more frequently.
For the conventional non-volatile semiconductor memory device, however, it is difficult to sufficiently boost such a reduced power supply voltage. Therefore, it is necessary to increase the capacitance of the capacitor included in the booster. This requires either the use of a special capacitance insulating film such as a ferroelectric film or increase in area of the capacitance insulating film. However, using a special insulating film in the capacitor alone would make the process of forming the capacitor more complex than the process of forming a storage element, a logic element and the like. Therefore, it is difficult from the standpoint of manufacturing costs to use such a special capacitance insulating film. Moreover, increasing the area of the capacitor would impede reduction in size of the non-volatile semiconductor memory device.
As described above, in the conventional non-volatile semiconductor memory device, it is difficult to obtain a capacitor of a booster which enables reduction in size and voltage.